1. Field of the Invention
The present invention relates generally to material forming and more specifically to an apparatus and method for forming a workpiece at an elevated temperature.
2. Description of Related Art
Various methods are known for forming a metal sheet into a part or component. One method involves a draw process wherein a punch pulls a portion of the metal sheet into a die set and presses the metal sheet into a die cavity of the die set to form the part. During the process, the metal sheet typically undergoes a reduction or change in the cross-sectional area or wall thickness of the sheet. Such processes are typically limited by the material's ability to be strained past its rupture point. Thus, depending upon the complexity of the part, the forming stresses on the metal sheet during the forming process may result in metal failure or fatigue and correspondingly an unusable or scrap part.
Superplastic forming is a process that takes advantage of a material's superplasticity or ability to be strained past its rupture point under certain elevated temperature conditions. Superplasticity in metals is defined by very high tensile elongation and is the ability of certain materials to undergo extreme elongation at proper temperature and strain rate. Superplastic forming is a process used to produce parts that are difficult to form using conventional fabrication techniques.
During the superplastic forming process, the metal sheet, or as often referred to, the blank, is heated to a point of superplasticity and placed in a heated die set. The heated blank is clamped in the heated die set and predefined gas pressure is applied to one side of the sheet. The pressure forces the sheet into a die cavity of the female die while maintaining a target strain rate for deforming the sheet throughout the forming cycle. The superplasticity of the material enables forming of complex components that normally cannot be formed by conventional room temperature metal forming processes. Use of a superplastic forming process enables forming a workpiece with a deep cavity or one formed over very small radii. Superplastic forming does have a disadvantage in that it normally requires relatively long forming cycle times. Specifically, a conventional superplastic forming process used to manufacture a complex part can require a forming cycle time as high as 30 minutes.
Superplastic forming offers several advantages over conventional stamping techniques including increased forming strains, zero springback and very low tooling costs. However, superplastic forming often requires slow forming rates, which can make the process economically unfeasible for many applications. One process for forming a part from a metal sheet using superplastic forming includes using a preform punch to impart an initial generic shape to the metal sheet prior to applying gas pressure to complete the forming process by forcing the metal sheet into the die cavity of the female die to form the part.